Seismic-Stratigraphic Analysis of Miocene System, Offshore Texas--Models and Implications

W. C. Riese, W. A. Hill, R. N. Rosen

The application of traditional seismic-stratigraphic models to the Miocene System offshore Texas was tested and found to be inadequate for the description of this section. Although the basic principles inherent to the application of seismic stratigraphy are useful, there are significant deviations from the model geometries of system tracts in the Miocene System. The most significant of these discrepancies is the apparent absence of lowstand wedges and shelf-margin wedges.

Problems with applying the traditional seismic stratigraphic models and geometries to the interpretation of this section have been recognized by others, and alternative ramp and growth-fault models have been suggested. These, too, appear to be inadequate for interpretation of this section: the ramp model fails to account adequately for outer neritic bathymetries in apparent outer-shelf settings during lowstands; the growth-fault model fails to adequately explain downthrown expansion of predominantly shale intervals.

The alternatives proposed postulate a fundamental difference in global, or at least basinal, water budgets for Miocene time relative to the Pleistocene or Holocene: eustatic levels in the Gulf of Mexico during the Miocene were apparently several hundred feet higher than during the Pleistocene and therefore erosion of the shelf during lowstands was minimal. We may also infer that surface gradients on the Texas shelf were steep during the Miocene and there was no pronounced continental shelf-slope break. Without such a break there is no steep surface against which to onlap the updip reaches of shelf-margin wedges or lowstand wedges, thus accounting for the absence of these geometries in our seismic data. This accounts for the presence of depositional fans in outer neritic, apparently shelfal settings during lowstands.

Expansion of shale-prone section downthrown to growth faults is accounted for by noting the proximity of what must have been a broad zone of structural foundering on the outer shelf to cold, nutrient-rich, deep-basin waters. As sea levels rose during highstands to further transgress an already submerged shelf, upper bathyal ecozones were brought onto the "shelf," expanding the geographic limits of cold-water organisms. More critically, this brought nutrient-rich waters to a broader reach of photic-zone organisms. The proliferation of these organisms accounts for the expansion of shale-prone intervals along growth faults; these sections are enriched in fossil content.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.